The invention is related to an apparatus and method for dispensing coatings. In particular, the invention is related to a delivery system that includes at least one pressure vessel within which is stored a deliverable substance having a coating component interspersed with a fluid component.
The use of volatile organic compounds as carriers for the delivery of coatings is well-known. However, increasingly there is a need for environmentally friendly carriers which minimize the use of organic carriers such as organic solvents. Supercritical fluids have emerged as such a viable carrier in coating applications, particularly in applications requiring the delivery of a substance in spray form. While supercritical fluids are known to have solvating powers similar to organic solvents, they also present advantages over organic solvents because of their higher diffusivities, lower viscosities, and lower surface tensions.
A supercritical carrier may be considered any compound at a temperature and pressure above certain critical values of temperature and pressure. The critical temperature of a compound is the temperature above which the pure compound in gaseous state cannot be converted to a liquid, while a compound""s critical pressure is the vapor pressure of the pure compound in gaseous state at the critical temperature. The critical point of the compound occurs at the temperature and pressure at which the gas and liquid phases are no longer separately defined, but instead a fluid exists in a state that is considered neither liquid nor gas. In the supercritical state, a fluid confers the carrier properties expected from a liquid while at the same time providing transport characteristics expected from gases.
Various compounds are known to exist as supercritical fluids, including ethylene, carbon dioxide, ethane, nitrous oxide, propane, and even methanol and water. The low cost and ready availability of supercritical carbon dioxide have made it a popular choice for a variety of applications. Also, with its critical temperature of 31.1xc2x0 C., critical pressure of about 73 atm, and critical density of about 470 kg/m3, supercritical carbon dioxide has properties amenable to applications using standard pressure vessel technology.
Various applications have been explored for supercritical carriers, including use in the delivery of protective coatings to various commercial building substrates such as marble, stone, cast stone, architectural terra cotta, concrete, and concrete block. The degradation of such materials due to pollution, acid rain, and other destructive forces can be substantially decreased if a relatively thin protective coating is applied.
Several supercritical fluid technologies have been disclosed by investigators. For example, U.S. Pat. No. 4,923,720 to Lee et al. is directed to the use of supercritical fluids as diluents in the liquid spray application of coatings. A process and apparatus for coating substrates is provided in which a supercritical fluid, such as supercritical carbon dioxide fluid, is used as a viscosity reduction diluent for coating formulations.
However, prior art methods and devices for applying coatings using supercritical fluids suffer from complexity and concomitant bulky equipment, rendering the technology inconvenient to use and inaccessible to many potential customers. Commercial and laboratory equipment for applying coatings using supercritical fluids generally fall into two classes, batch and continuous. Typically, the main storage element of prior art batch systems is a floating piston accumulator. The coating material and supercritical fluid are held captive on one side of the piston, while the pressurization fluid is stored on the other. In such systems, the coating material and CO2 are added at a pressure typically above 1000 psi so that the CO2 remains in a liquid state. Such an arrangement requires high-pressure pumps. After the desired amounts of coating material and CO2 have been added, the two components must be mixed. Mixing usually is effected by circulating material in and out of the piston accumulator. The pressurizing fluid, disposed on the other side of the piston accumulator, is used to effect transport of the deliverable substance through a hose to a spray nozzle. Such batch systems are heavy due to the weight of the piston accumulator, high-pressure pumps, and associated controls. The weight of commercial units ranges between about 3000 lbs and about 1500 lbs. for equipment capable of delivering 6 kgs per batch, not including the CO2 supply bottle.
Continuous systems typically require two or three high-pressure pumps, along with complex flow meters and controls for accurately metering and mixing the coating material and supercritical fluid components. Multiple control loops and a programmable logic controller may be required. Such systems are less common, due to the required level of sophistication of controls. Further, although the commercial, continuous systems are capable of supplying about 100 grams to about 300 grams per minute of deliverable product, they are heavy, typically weighing between about 180 lbs. and 1000 lbs.
The above-described batch and continuous systems are heavy, bulky, require multiple high-pressure pumps, and require heavy CO2 cylinders with high stored energies. These systems also require significant equipment maintenance, as well as an additional energy source to power pumps and controls.
Thus, there exists a need for an improved apparatus and an improved method for dispensing coatings using supercritical fluids. There also exists a need for an apparatus with simplicity in design, compactness, and portability so that the device may be manually transported. Moreover, there exists a need for methods and devices that can deliver coatings with controllable composition and thickness.
The invention is related to a coating delivery system including a first pressure vessel operable in sealed and unsealed conditions, a deliverable substance having a coating component interspersed with at least one of liquefied carbon dioxide and supercritical carbon dioxide, and a pressurizing fluid. The deliverable substance is disposed in the first pressure vessel. The pressurizing fluid is provided (1) at a pressure greater than the vapor pressure of carbon dioxide if the deliverable substance includes liquefied carbon dioxide, or (2) at a pressure greater than the critical pressure of carbon dioxide if the deliverable substance includes supercritical carbon dioxide. In addition, the pressurizing fluid directly exerts a pressure on the deliverable substance to effect transport thereof when the first pressure vessel is in the unsealed condition.
The first pressure vessel may include a dip tube, a head region, and a deliverable substance region, with the dip tube being disposed in the first pressure vessel and having an inlet proximate the deliverable substance region. The head region of the first pressure vessel may serve as an accumulator for accommodating changes in volume. In some embodiments, a second pressure vessel may be provided, with the pressurizing fluid being disposed in the second pressure vessel in communication with a head region of the first pressure vessel. A regulator may be provided for regulating the transport of pressurizing fluid from the second pressure vessel to the first pressure vessel, and a temperature control system may be provided for changing the temperature of the deliverable substance. The temperature control system may include induction coils, resistive heating coils, or fluid circulation coils.
The coating delivery system optionally may include a circulation loop for recirculating the deliverable substance. The circulation loop may include a pump and a check valve, with the check valve permitting flow of deliverable substance in one direction in the circulation loop. The circulation loop may terminate inside the first pressure vessel remote from the head region. A temperature control system also may be included for changing the temperature of the deliverable substance exiting the pump.
In some embodiments, at least one agitation component such as a magnetic stirrer or at least one mixing ball may be provided for agitating the deliverable substance. The coating delivery system also may include a heated dispensing hose communicating with the first pressure vessel, with the hose having an orifice, spray attachment, or other nozzle attachment for dispensing the deliverable substance.
The coating component may be a polymeric compound. In some embodiments, the coating component may be an enamel, an alkylsilicone resin, or a fluorinated resin. The deliverable substance may include a vapor component, and the pressurizing fluid may be substantially less dense than the vapor component. The pressurizing fluid may be a gas selected from the group consisting of hydrogen, helium, nitrogen, air, oxygen, argon, and methane. In some embodiments, the pressurizing fluid may be a gas selected from the group consisting of helium or hydrogen. The first pressure vessel may have a pressure between about 200 psi and about 4500 psi. A circulation loop may be provided for recirculating deliverable substance, with the circulation loop terminating inside the first pressure vessel remote from a head region thereof.
The invention also is related to a method of applying a coating to a substrate including: providing a deliverable substance in a first pressure vessel, the deliverable substance comprising a coating component interspersed with at least one of liquefied carbon dioxide and supercritical carbon dioxide; providing a first fluid capable of applying pressure to the deliverable substance; allowing the first fluid to directly apply pressure to the deliverable substance (1) at a pressure at least the vapor pressure of carbon dioxide if the deliverable substance comprises liquefied carbon dioxide, or (2) at a pressure at least the critical pressure of carbon dioxide if the deliverable substance comprises supercritical carbon dioxide; delivering the deliverable substance to the substrate.
In some embodiments, the first fluid may be provided in a second pressure vessel that communicates with the first pressure vessel. The method may additionally include heating the deliverable substance prior to spray discharge, pumping the deliverable substance, agitating the deliverable substance, and/or recirculating a portion of the deliverable substance. When the deliverable substance is delivered to the substrate, fluid in a head space of the pressure vessel may remain substantially undisturbed. In addition, the method may include heating the deliverable substance in the first pressure vessel to cause a change in composition of the deliverable substance.